1. Field of the Invention
The present invention relates to a sheet post-processing apparatus which discharges a folded sheet bundle to be stacked in imbricated state and an image forming apparatus having the same.
2. Description of Related Art
Some of image forming apparatuses of the related art forming images on sheets, such as copying machines and laser beam printers, have a sheet post-processing apparatus which bundles the image-formed sheets, performs the binding process for the sheet bundle and the folding process, and make a booklet.
Such sheet post-processing apparatus stacks the image-formed sheets onto a tray, bundles them, and performs the binding process at the substantially center vicinity of the sheet bundle. The substantially center vicinity of the sheet bundle is pushed out by a pushout member. The sheet bundle is pushed into between the nips of a pair of folding rollers. While being conveyed by the folding rollers, the sheet bundle is performed the folding process. The strengthening process of the folded end of the sheet bundle is performed. The sheet bundle is discharged and stacked onto a folded sheet bundle tray.
In the case of discharging and stacking the sheet bundle onto the folded sheet bundle tray, as illustrated in FIG. 20, a sheet bundle P1 stacked onto a folded sheet bundle tray 840 is located outside the moving region of a press unit 860 while performing the folded end process for the next sheet bundle P2. The sheet bundle P1 is returned to near a pair of folding conveying rollers 812 before the sheet bundle P2 performed the folded end process is discharged.
In Japanese Patent Application Laid-Open No. 2008-184311, the amount of return is determined according to the type of sheet used so that the leading end of the sheet bundle P2 is located on downstream in the discharge direction of the trailing end of the sheet bundle P1 when the sheet bundle P2 is discharged. The sheet bundle P2 is stacked in imbricated state, thereby preventing sheet jamming and sheet folding.
In the sheet post-processing apparatus of the related art, when the leading end of the sheet bundle P2 is abutted onto the sheet bundle P1, the conveyance of the sheet bundle P1 to downstream in the discharge direction has not been started. In the case of discharging the sheet bundle P2 which is a sheet bundle including sheets forming it which have low rigidity and a few number of sheets, the sheet bundle P2 can easily hang down in the substantially vertical direction. As illustrated in FIGS. 18 and 19, the leading end of the sheet bundle P2 is abutted onto the sheet bundle P1, the sheet bundle P2 is bent and rounded, and stacking failure can be caused.
The present invention provides a sheet post-processing apparatus which can reliably stack a sheet bundle in imbricated state without bending it and an image forming apparatus having the same.